The invention relates to dual wheel caster assemblies, and more particularly to a caster assembly with improved stability for swivel mounted support wheels on movable vehicles such as infant strollers, baby carriages, carts, or wheelchairs.
Prior art single and dual wheel caster assemblies have distinct problems. A single wheel swivel-mounted to a rigid frame tends to flutter and tracks poorly when exposed to lateral forces. While dual wheel casters assemblies tend to track better, they are still subject to annoying flutter. The flutter in these casters doesn't affect the directional stability of the load, but rather creates an annoying vibration and adds drag to the forward motion of the system.
The flutter occurs because the force correcting the direction of the caster is roughly proportional to the sine of the angle between the direction of travel of the system and the direction the caster is pointing. Since the sine of zero is zero, there is very little restoring force for small angular deviations and small perturbations lead to oscillations of the caster about the direction of travel. The light weight of the caster system in comparison to the load means its natural frequency of oscillation is high compared to normal directional pertubations of the load, leading to a flutter of the wheels rather than deviation of cart direction.
U.S. Pat. No. 3,795,408 teaches use of a frictional clutch system for the purpose of damping "fishtailing" oscillations in trailer systems, such as a truck towing one or more trailers. In that case trailers being towed out of a curve are apparently over steered or under steered leading to a side to side oscillation of increasing amplitude, particularly under acceleration. In this system the oscillation is driven by motions of the load (trailer) in a whip like fashion, driven by over and under steering corrections created by the trailer geometry. In this system the energy of the oscillation is contained in the motions of the load rather than the deviation of the wheels. Frictionally restricting relative rotation between the wheels being pivoted to steer the trailer acts to dampen this motion by removing the energy of the oscillation and thus prevents fishtailing.
In dual wheel caster assemblies the wheels are generally free to rotate independent of one another. While this rotational freedom is necessary for adequate directional maneuverability, it turns out that flutter in the dual wheel systems can be prevented by binding the wheels to each other in some fashion so that they are constrained to rotate as a unit. It is therefore desireable to have a system that allows adequate rotational freedom for directional maneuverability while still providing enough rotational coupling to prevent flutter.
A prior commercial solution to this problem has been to add interference ribs in the hubs of each wheel to create frictional coupling between the wheels and the axle. Such ribs run axially, and project radially into the ID of the wheel. Their purpose is to provide an interference fit, which ties together the motion of the wheels in a dual wheel caster system. The system has several drawbacks. The first drawback is the tolerances. Close tolerances in the plastic(s) used in wheel fabrication are difficult to hold. This can cause the wheel to be too loose or too tight on the axle causing flutter or making it hard to steer (respectively). The ribs help to reduce this to some degree, but the problem still exists. Since this system of wheel attachment is used in both wheels in each caster system, the tolerances must be held in both wheels to provide the proper resisting torque to eliminate wheel flutter while not being too great as to hamper maneuverability.
Another problem with such prior art devices is that the ribs can wear down with use. Thus, as the ribs wear down, wheel flutter will tend to return.